The removal of carbon dioxide from mixed gas streams is of great industrial importance and commercial value. Carbon dioxide is a ubiquitous and inescapable by-product of the combustion of hydrocarbons and there is growing concern over its accumulation in the atmosphere and its potential role in global climate change. Laws and regulations driven by environmental factors may therefore soon be expected to require its capture and sequestration. While existing methods of CO2 capture have been satisfactory for the scale in which they have so far been used, future uses on the far larger scale required for significant reductions in atmospheric CO2 emissions from major stationary combustion sources, such as power stations fired by fossil fuels, makes it necessary to improve the energy efficiency of the processes used for the removal of CO2 from gas mixtures and thereby lower the cost of CO2 capture. According to data developed by the Intergovernmental Panel on Climate Change, power generation produces approximately 78% of stationary source emissions of CO2 with other industries such as cement production (7%), refineries (6%), iron and steel manufacture (5%), petrochemicals (3%), oil and gas processing (0.4%) and the biomass industry (bioethanol and bioenergy) (1%) making up the bulk of the total, illustrating the very large differences in scale between power generation on the one hand and all other uses on the other. To this must be added the individual problem of the sheer volumes of gas which will need to be treated. Flue gases generally consist mainly of nitrogen from combustion air, with the CO2, nitrogen oxides, and other emissions such as sulfur oxides making up relatively smaller proportions of the gases which require treatment. Typically, the wet flue gases from fossil fuel power stations typically contain about 7-15 vol % of CO2, depending on the fuel, with natural gas giving the lowest amounts and hard coals the highest.
Cyclic CO2 sorption technologies such as Pressure Swing Absorption (PSA) and Temperature Swing Absorption (TSA) using liquid sorbents are well established. The sorbents mostly used include liquid solvents, as in amine scrubbing processes, although solid sorbents are also used in PSA and TSA processes. Liquid amine sorbents dissolved in water are probably the most common sorbents. Amine scrubbing is based on the chemical reaction of CO2 with amines to generate carbonate/bicarbonate and carbamate salts: the aqueous amine solutions chemically trap the CO2 by the formation of one or more of these ammonium salts (carbamateicarbonate/bicarbonate). The reaction tends to be reversible, and these salts can be converted back to the original components upon suitable adjustment of conditions, usually temperature, enabling the regeneration of the free amine at moderately elevated temperatures. Commercially, amine scrubbing typically involves contacting the CO2 and/or H2S containing gas stream with an aqueous solution of one or more simple alkanolamines which are selected preferentially, as the hydroxyl group confers greater solubility in water for both the amine(s) and for the reaction product(s). Alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), as well as a limited set of hindered amines, are currently used in commercial processes. The cyclic sorption process requires high rates of gas-liquid heat exchange, the transfer of large liquid inventories between the sorption and regeneration zones, and high energy requirements for the regeneration of amine solutions. With the exception of the amine processes based on hindered amines, the amine scrubbing process is challenged by the corrosive nature of the amine solutions containing the sorbed CO2, which forms the amine-CO2 reaction products. Without further improvement, these difficulties would limit the economic viability of the aqueous amine scrubbing processes in very large scale applications.
The cyclic sorption processes using aqueous sorbents typically require a significant temperature differential in the gas stream between the sorption and desorption (regeneration) parts of the cycle. In conventional aqueous amine scrubbing methods, relatively low temperatures, e.g., less than 50° C., are required for CO2 uptake, with an increase to a temperature above about 100° C. (e.g., 120° C.) required for the desorption. The heat required to maintain the thermal differential is a major factor in the cost of the process. With the need to regenerate the solution at temperatures above 100° C., the high latent heat of vaporization of the water (˜2260 kJ/Kg at ˜100° C.) obviously makes a significant contribution to the total energy consumption. If CO2 capture is to be conducted on the larger scale appropriate to use in power plants, more effective and economical separation techniques need to be developed.
Another area where more efficient CO2 separation processes are needed is in enhanced oil recovery (EOR), where CO2 is re-injected into the gas or liquid hydrocarbon deposits to maintain reservoir pressure. With the advanced age of many producing reservoirs worldwide and the ever-increasing challenge of meeting demand, the expanding use of EOR methods is becoming more widespread. Typically, the source of carbon dioxide for EOR is the producing hydrocarbon stream itself, which may contain anywhere from less than 5% to more than 80% of CO2. Other options are capture of CO2 from the flue gases of various combustion sources and pre-combustion capture of CO2 from shifted syngas produced in fuel gasification processes.
The use of sterically hindered amines for CO2 capture was proposed by Sartori and Savage in “Sterically Hindered Amines for CO2 Removal from Gases,” Ind. Eng. Chem. Fundamen., 1983, 22(2), 239-249, pointing out that sterically hindered amines have unique capacity and rate advantages in CO2 sorption processes: their rich solutions can be desorbed to a greater extent than their non-substituted counterparts, thus producing a leaner solution (lower total carbamate/bicarbonate/carbonate concentration), which tends to result in a greater mass transfer upon reabsorption. A limited number of processes using sterically hindered amines as alternatives to MEA, DEA, and TEA are used commercially for CO2 capture; examples include the KS-1™ Process from Mitsubishi Heavy Industries and Kansai Electric Power Co and the ExxonMobil Flexsorb® Process, which uses sterically hindered amine(s) for selective H2S separation. Processes using solid sorbents are also known: they may avoid some of the limitations of amine scrubbing, such as large capital investment and high regeneration energy intensity, but they suffer from a lack of sorbents having sufficiently selective CO2 sorption under the humid conditions present in combustion flue gas and from the difficulty in designing gas/solid contactors to process large volumes of gas at high throughput rates.
In the design of a practical CO2 capture process, a number of issues should be considered, including the efficiency of the capture process in terms of the cyclic capacity of the process, the efficiency of the capture process in terms of the energy required for desorption of the CO2 and regeneration of the amine sorbent, the requirement for steady replenishment of fresh amine to maintain the desired sorption capacity, as well as corrosion factors. These issues are, of course, directly affected by the chemistry of the sorption process and the efficiency of the chemisorption processes. As such, conventional aqueous amine scrubbing processes are dependent in part on the ability of the sorbent medium to react with the CO2 and on the rate of that reaction. Another important factor is the optimum combination of conditions for sorption and desorption of CO2. This is one of the key parameters that defines the cyclic capacity (which is also known as the swing capacity or working capacity of the amine scrubbing process). The molar ratio of the CO2 capture per mole of amine is one factor that can determine the cyclic capacity. Another factor is the practical concentration of the amine in the solution. More CO2 per unit of amine can be captured as the concentration of the amine increases, and, in turn, a more concentrated solution of amine per unit volume of solution confers several benefits. First, less liquid amine solution needs to be circulated between the sorber and stripper zones as the carrying capacity increases with increasing amine concentration. Second, a more concentrated amine solution requires less energy to regenerate the amine and release the CO2 in the stripper zone, since less water enters the stripper.
In conventional aqueous amine systems, the process by which CO2 is sorbed by tertiary amines is believed to proceed by dissolution of the gaseous CO2 in water to form carbonic acid (H2CO3), which is neutralized by the amine to form an ammonium bicarbonate. At high pH, the ammonium bicarbonate may then react with a second mole of amine to form an ammonium carbonate. Primary and secondary amines may also react directly with the CO2 to form an ammonium carbamate, which is itself stable in the presence of water and may be present as a significant reaction product, especially at high amine concentration; tertiary amines, lacking a free proton, are incapable of forming the carbamate, which is believed to be formed by primary and secondary amines by the initial formation of an unstable zwitterion intermediate, which rapidly decomposes via internal proton transfer to the carbamic acid. Both the zwitterion and the carbamic acid are unstable, and it is not known which equilibrium form undergoes further reaction, although it is posited that it is the carbamic acid, which may be deprotonated by a second equivalent of free amine to produce the ammonium carbamate salt with the overall stoichiometric requirement of two moles of amine per one mole of carbon dioxide sorbed (0.5:1 CO2:amine).
Further reaction of the carbamate with water may lead to a final bicarbonate product with a ˜1:1 CO2:amine ratio, or to a carbonate product with a ˜0.5:1 CO2:amine ratio depending on solution pH. Thus, the conventional aqueous amine processes, which use primary amines, can have a limited sorption efficiency, which has a maximum CO2:amine molar of ratio of 1:1 achieved with the formation of the bicarbonate as the final reaction product.
While the use of hindered amines for CO2 removal might therefore present itself as an attractive approach, operational problems and difficulties have been encountered. For example, although the sterically hindered primary amine 2-amino-2-methyl-1-propanol (AMP, whose use for CO2 sorption is reported by Sartori et al., op. cit.), forms a bicarbonate from which the free amine is readily thermally released, it has been found that its practical utility is limited by the formation of insoluble precipitates at amine concentrations above about 3M in aqueous solution. For this reason, the solution has to be relatively dilute so that the physical size of the unit for a given treatment capacity has to be rather large to handle the higher volume of liquid solvent, resulting in a larger capital investment, a lower operating efficiency, and a higher cost for the CO2 capture.